Cathodes can emit electrons by photoemission, thermionic emission, and field emission, or as the result of negative electron affinity. A field-emission cathode (or field emitter) supplies electrons when subjected to an electric field of sufficient strength. The electric field is created by applying a suitable voltage between the cathode and an electrode, typically referred to as the anode or gate electrode, situated a short distance away from the cathode.
When used in a flat-panel display such as a flat-panel television or video monitor, a field emitter typically contains a group, often a very large group, of individual electron-emissive elements distributed across a supporting structure. This configuration is referred to here as an area field emitter. Busta, "Vacuum microelectronics--1992," J. Micromech. Microeng., Vol. 2, 1992, pp. 43-74, describes a number of different techniques that have been investigated for manufacturing electron-emissive elements in gated area field emitters.
Fischer et al, "Production and use of nuclear tracks: imprinting structure on solids," Rev. Mod. Phys., October 1983, pp. 907-948, describes how nuclear tracks are employed in manufacturing field emitters according to a replica technique. In Fischer et al, nuclear tracks are formed through a substrate. The tracks are etched to create cavities in the substrate after which metal is deposited on the substrate to create a film that extends across the substrate and fills the cavities. The substrate is then removed. The metal film, including the resultant metal protrusions, form an area field emitter as a replica of the substrate.
Some area field emitters employ elongated electron-emissive elements. For example, Yoshida et al, U.S. Pat. No. 5,164,632, discloses a gated field emitter in which solid elongated electron-emissive elements are created in pores extending through a dielectric layer. Greene et al, U.S. Pat. No. 5,150,192, uses hollow elongated electron-emissive elements.
Other area field emitters utilize generally conical electron-emissive elements. See Spindt et al, U.S. Pat. No. 3,665,241. Also see Borel, U.S. Pat. No. 4,940,916; Betsui, "Fabrication and Characteristics of Si Field Emitter Arrays," Tech. Dig. IVMC 91, pp. 26-29; and Fukuta et al, European Patent Publication 508,737 A1.
In yet other area field emitters, electron-emissive particles of various shapes and/or sizes are distributed across a supporting layer at the bottoms of openings that extend through a gate structure overlying the supporting layer. Chason, U.S. Pat. No. 5,019,003, discloses an example of this type of field emitter. Other such examples are disclosed in Thomas et al, U.S. Pat. No. 5,150,019; Jaskie et al, U.S. Pat. No. 5,278,475; and Kane et al, U.S. Pat. No. 5,252,833.
When a portion of a gated area field-emission device in a flat-panel CRT is actively emitting electrons as the result of a suitable applied extraction voltage, the current density produced by emitted electrons ideally should be uniform across the activated portion. In a real field emitter, the emission current density typically becomes more uniform as the emitter packing density--i.e., the number of electron-emission elements per unit area--increases and, correspondingly, as the lateral area occupied by an electron-emissive element decreases.
In manufacturing high-quality prior art gated electron emitters, use of technologies such as photolithography typically places severe restrictions on the minimum lateral size of electron-emissive features such as an electron-emissive element or an opening for an electron-emissive element, especially in a volume production environment. More specifically, depth of field, sometimes referred to as depth of focus, is commonly employed in characterizing radiation-based patterning techniques such as photolithography. Briefly stated, the depth of field is the (maximum) distance, measured along the optic axis, across which an acceptable pattern can be obtained on a generally flat surface situated, generally orthogonal to the optic axis, at any point along that distance.
The depth of field in photolithography is finite and, in particular, is relatively small compared to what would be desirable for efficient manufacturing of area electron emitters on a production scale. Consider an electron-emitting device in which the total area of the surface to be photolithographically patterned is several square centimeters or more. The flatness of the surface being patterned, the presence of features on the surface, and the alignment of the surface in the photolithographic radiation-exposure combined with the small photolithographic depth of field significantly limit the minimum lateral size of features photolithographically defined at the surface using a single radiation exposure.
Finer photolithographic patterns can be obtained by exposing small parts of the total area to the patterning radiation in separate expose-and-move steps. However, such an expose-and-move process is time-consuming and therefore expensive because it requires re-alignment and re-focus before each exposure.
As an example, the conical electron-emissive elements in Betsui and Fukuta et al appear to have a photolithographically defined base diameter of 1-3 .mu.m. It is desirable to overcome these limitations so as to be able to fabricate high-quality gated area electron emitters having smaller lateral electron-emissive features. It is also desirable to increase the emitter packing density so as to attain more uniform emission current density.